Compound semiconductors are compounds which act as a semiconductor by a combination of two or more elements, rather than by single elements like silicon or germanium. Currently, a variety of such compound semiconductors have been developed and are being used in various applications. Typically, compound semiconductors may be used in thermoelectric conversion devices using the Peltier effect, light emitting devices (such as light emitting diodes and laser diodes) and solar cells using the photoelectric conversion effect, and the like.
First, solar cells are being researched actively as an alternative energy source for the future because they need no additional energy sources other than solar light, which exists naturally and is thus eco-friendly. Solar cells can be classified into silicon solar cells using single elements (mainly, silicon), compound semiconductor solar cells using compound semiconductors, tandem solar cells in which two or more solar cells having a different bandgap energy are stacked, and the like.
Among them, compound semiconductor solar cells use compound semiconductors in a light absorbing layer which absorbs solar light to generate electron-hole pairs; especially, such compound semiconductors may be III-V group compound semiconductors such as GaAs, InP, GaAlAs and GaInAs, II-VI group compound semiconductors such as CdS, CdTe and ZnS, and compound semiconductors typically represented by CuInSe2.
A light absorbing layer in solar cells needs to exhibit superior long-term electric and optical stability and high photoelectric conversion efficiency, and to allow for ease adjustment of bandgap energy and conductive type through compositional changes or doping. A light absorbing layer also needs to satisfy other requirements such as manufacturing costs and yields in order to be commercialized. However, conventional chemical semiconductors have failed to meet all such requirements at once.
Also, thermoelectric conversion devices can be applied in thermoelectric conversion power generation, thermoelectric conversion cooling and the like, and are generally made up of n-type thermoelectric semiconductors and p-type thermoelectric semiconductors, with both being connected electrically in serial and thermally in parallel. Among them, thermoelectric conversion power generation uses thermal electromotive force generated by maintaining a temperature difference in a thermoelectric conversion device, converting thermal energy into electric energy. And thermoelectric conversion cooling uses the effect of a temperature difference being generated at opposite ends of a thermoelectric conversion device by flowing a direct current through the opposite ends, converting electric energy into thermal energy.
The energy conversion efficiency of such thermoelectric conversion devices can depend broadly on a figure of merit of thermoelectric conversion devices, which is ZT. Here, ZT can be determined by Seebeck coefficient, electrical conductivity, thermal conductivity and the like; a higher ZT means a thermoelectric conversion material having higher performance.
Though numerous thermoelectric conversion materials have been suggested so far, thermoelectric conversion materials with high thermoelectric conversion performance have yet to be arranged adequately. Especially, applications for thermoelectric conversion materials are gradually being widened in recent years, and temperature conditions can differ depending on such applications. However, since thermoelectric conversion materials may exhibit different thermoelectric conversion performance depending on temperature, it is necessary that thermoelectric conversion performance of given thermoelectric conversion materials should be optimized for the applications in which they are applied. Still, thermoelectric conversion materials have yet to be arranged adequately to have performance optimized for various ranges of temperature.